1. Field of the Invention
The present invention relates to a three-phase active filter for compensating undesirable harmonic deviations in AC energy delivered from a three-phase AC source to a three-phase load. More particularly, the invention relates to such an active filter having a control scheme utilizing vector control principles.
2. Description of the Prior Art
In a three-phase power distribution system wherein a three-phase AC source is feeding a three-phase load, undesirable harmonic deviations are often generated on the interconnecting supply line. These harmonic deviations result in harmonic distortion which can cause problems in the reliable operation of harmonic sensitive components in the system. Particularly, non-linear loads such as rectifiers, power converters, and other types of power electronic equipment may draw rectangular or pulsed currents from the source which consist of a fundamental component and a series of harmonics. The current harmonics may create a voltage drop across the source impedance which results in harmonic distortion of the voltage seen by the load.
To reduce these harmonics and the concomitant distortion, various active power filter arrangements have been utilized. Originally, such filters generally fell into one of two categories: (1) those using controllable impedance or admittance as the active element, and (2) those using linear power amplifiers controlled by various feedback techniques. The first type had relatively low efficiency and was thus considered not well suited for power applications. The second type tended to be cost prohibitive in relatively high power applications.
A third type of active filter was later developed to overcome some of these deficiencies. This filter utilizes a current or voltage "generator" selectively controlled to produce a flow of energy matching and compensating the harmonic components to be eliminated. A nearly ideal filter characteristic is thus produced at the point of connection into the system. Filters of this type in single-phase systems are disclosed and described in U.S. Pat. No. 3,825,815, issued Jul. 23, 1974 to Gyugyi et al. Three-phase power distribution systems utilizing such filters are illustrated in U.S. Pat. No. 4,651,265, issued to Stacey et al. on Mar. 17, 1987. Both of these patents are incorporated herein by reference.